The present invention relates to a projection optical system for a liquid crystal projector for projecting a picture displayed on a liquid crystal display (LCD) panel onto a large screen, and more particularly, to a projection optical system for a liquid crystal projector whose volume is significantly reduced in terms of lens diameter and thickness, so that the manufacturing cost can be saved.
Recently, with the advent of semiconductor integration technology, high density integration has been successively achieved, so that liquid crystal displays have seen wider use (especially in the field of small portable liquid crystal televisions) which have not experienced serious problems as to their application and manufacture. However, as for large-screen television displays, a thin film transistor liquid crystal display (TFT LCD) using a twisted nematic liquid crystal as a light controlling material, and a TFT as a switching element of pixels, requires a quite difficult manufacturing process. Moreover, the larger the TFT LCD becomes, the less the yield becomes. Therefore, there is a limit in attaining a large screen with a TFT LCD. Accordingly, in order to obtain a large screen with a TFT LCD, the TFT LCD is adopted to a projector by which the large screen can be attained easily, as an image processor for controlling the light incident from a light source by an electrical image signal.
FIG.1 shows a schematic diagram of a projection optical system for a conventional liquid crystal projector composed of multiple lenses. Also, FIG.2 schematically shows the arrangement structure of optical units of a conventional liquid crystal projector.
To ensure the proper distance between a lens arrangement 30 and the screen, the conventional liquid crystal projector should be in the form of a retrofocus optical system in which a "back focal" length, i.e., from the final lens surface (R.sub.13) of the lens arrangement to the screen, is relatively long.
As shown in FIG. 1, the projection optical system for the conventional liquid crystal projector has six lenses in total. Here, a first lens is a plano-convex lens having both surfaces whose curvature radii are R.sub.1 and R.sub.2, respectively, and a second lens is a meniscus lens having both surfaces whose curvature radii are R.sub.3 and R.sub.4, respectively. The first and second lenses form a front lens portion 31. A third lens is a concave lens whose curvature radii are R.sub.6 and R.sub.7, a fourth lens is a convex lens having both surfaces whose curvature radii are R.sub.8 and R.sub.9, a fifth lens is a convex lens whose curvature radii are R.sub.10 and R.sub.11, and a sixth lens is a convex lens having both surfaces whose curvature radii are R.sub.12 and R.sub.13, respectively. The third to sixth lenses form a rear lens portion 32. A 20 is provided between the front lens portion 31 and the rear lens portion 32.
Referring to FIG.2, a projection optical system 30 is provided in front of a light emitting surface 46a of a prism 46 having dichroic filters 46c and 46d intersecting with each other by an angle of 45.degree.. Three LCDs 48a, 48b and 48c are provided in front of three light incident surfaces 46g, 46b and 46r, respectively. Red, green and blue beams obtained from a light source 41 are incident to the LCDs 48a, 48b and 48c, respectively. In front of the light source 41, an infrared filter 44a for filtering infrared rays from among the beams projected therefrom is provided. A first dichroic mirror 44b for screening and reflecting the green beam from among traveling light is provided in front of the infrared filter 44a. A first total reflector 42a for reflecting the green beam onto the green LCD 48a is provided in the traveling path of the green beam. A second dichroic mirror 44c for reflecting the blue beam onto the blue LCD 48b is provided in the traveling path of the beam transmitted via the first dichroic mirror 44b, i.e., the red beam. Also, second and third total reflectors 42b and 42c for reflecting the red beam onto the red LCD 48c is provided in the traveling path of the light beam transmitted via the second dichroic mirror 44c, i.e., the red and blue mixed beam.
In the prior art as described above, in view of the characteristics of the above liquid crystal projector, a greater contrast between the brightness of the projected image and the room illumination is often necessary. In addition, since the overall optical system constituting such a projector is necessarily a telecentric optical system, the effective diameter of rear lens portion 32 increases, which is a disadvantage in that a size reduction is difficult to achieve due to the larger lens size.
Now, the lens configuration in the conventional projection optical system will be described with reference to FIG. 1.
The first through fifth lenses having curvature radii R.sub.1 through R.sub.11 of the first five lenses, have symmetric Gaussian forms, and more particularly, have a configuration of a telecentric optical system. Also, stop 20 is placed between the front lens portion 31 and rear lens portion 32.
Also, a retrofocus type lens form is required so that a longer back focal length can be attained, and therefore the third convex lens is added (curvature radii R.sub.12 and R.sub.13) which causes a degree of dissymmetry in the Petzval sum. Consequently, curvature radii R.sub.4 and R.sub.6 are slightly different from those of lenses satisfying the conventional Gaussian form.
However, in achieving the above requirement (i.e., the retrofocus type lens form), another problem arises in that the lens diameter of rear lens portion 32 becomes greater. Also, since the back focal length is longer in such an optical system, the length of liquid crystal projector 30 is accordingly longer. Larger projectors lead to an inevitable rise in the manufacturing cost thereof.
Moreover, the focusing of the above-described liquid crystal projector is adjusted by varying a dimension D.sub.13. However, if the overall optical system is installed in a tube, it becomes necessarily difficult to manufacture and transport the resultingly large tube.